1. Field of the Invention
The present invention relates to a wire bonding apparatus.
2. Prior Art
One type of wire bonding apparatus includes a heating block, which is used to heat a workpiece, and a bonding horn which is positioned above this heating block so as to hold the capillary. Thus, the bonding horn is caused to undergo thermal expansion by the radiant heat from the heating block, and this results in a drop of the precision of the bonding position.
The structure disclosed, for example, in the Japanese Utility Model Application Publication (Kokoku) No. 5-396732 has been proposed as a means to prevent thermal expansion of the bonding horn. In this structure, a heat-shielding plate which is semicircular in cross section is installed so that the plate covers the circumferential surface of the lower part of the bonding horn. In addition, it is designed so that cooling air is blown between the bonding horn and the heat-shielding plate.
Further to the heating block, a wire bonding apparatus is generally equipped with: (a) an X-axis motor and Y-axis motor which drive an XY table and a bonding head in the X and Y directions, (b) a bonding horn which holds the capillary, (c) a Z-axis motor which causes the lifter arm to which the bonding horn is attached to pivot or move vertically, and (d) a bonding-load linear motor which applies a bonding load to the capillary so that the capillary presses the ball or wire against the bonding point.
The wire bonding apparatus of this type is disclosed, for example, in the Japanese Patent Application Laid-Open (Kokai) Nos. 4-317342 and 4-320350.
A wire bonding apparatus further includes a detection camera which detects the workpiece. Two types of cameras are known presently. In one type, a mirror tube which has a workpiece detection part is installed horizontally in an offset position with respect to the capillary, the detection camera is attached to the mirror tube, and the mirror tube is fastened to the bonding head. In another type, the detection camera is fastened to a camera supporting arm so as to be at an offset position with respect to a capillary, and the camera supporting arm is fastened to the bonding head. With the thus installed cameras, the bonding point is first detected by the detection camera, then the bonding coordinates are corrected, and then the capillary is driven in the X and Y directions and also in the Z direction. Wire bonding is performed after these movements.
In the prior art described above, countermeasures are taken only against the thermal expansion of the bonding horn that is caused by the radiant heat of the heating block. In other words, no consideration is given to the fact that the mirror tube and the camera supporting arm are also caused to thermally expand. The mirror tube and camera supporting arm are positioned above the heating block; therefore, they can easily undergo thermal expansion as a result of the thermal expansion of the heating block.
When the mirror tube and camera supporting arm thus undergo thermal expansion, the position of the point which is detected as the bonding point tends to shift, resulting in that a positional discrepancy occurs relative to the capillary. When a relative positional discrepancy thus occurs between the detected point and the capillary, the precision of the bonding position drops.
Furthermore, in the prior art described above, no consideration is given to other heat-emitting sources than the heating block. Accordingly, the bonding horn and the mirror tube or camera supporting arm may expand due to thermal expansion caused by heat emitted from the bonding horn, Z-axis motor and bonding-load liner motor, resulting in that positional discrepancies are generated in the capillary and workpiece detection part. In other words, even if thermal expansion of the bonding horn caused by the heating block is prevented in the conventional apparatuses, such is usually insufficient.
Meanwhile, the heat generated by the X-axis motor and Y-axis motor generally have almost no effect on the bonding horn, mirror tube or camera supporting arm.